This invention relates to a liquid crystal display device, and in particular to a liquid crystal display device of guest-host mode.
A liquid crystal display device of guest-host mode wherein a dye of high dichroic ratio is dissolved in a liquid crystal is relatively wide in visual angle, and, because of this advantage, is expected to be one of prospective display modes in future. To meet the demand for a color display device in recent years, the development for a color display device of guest-host mode has been also extensively undertaken.
As one example of such a color liquid crystal display device of guest-host mode, one wherein guest-host cells of three colors, i.e. yellow, cyan and magenta are superimposed one after another is known as suggested by T. Utida (Proc. 3rd. Display Res. Conf., p202, 1983).
In the liquid crystal display device of full color guest-host mode as mentioned above, it is requested for realizing a bright and clear color display that the spectrum of a dye therein indicates an ideal shape.
Further, the absorbancy of liquid crystal is required to be increased if a clear color is to be obtained, and, for the purpose of this, as much quantity of dye as possible must be dissolved into the liquid crystal. Accordingly, the solubility and dissolution stability of the dye to the liquid crystal are required to be improved. Furthermore, in order to enhance the contrast of display, it is necessary to improve the dichroic ratio as well as the degree of orientational order of the dye.
A terminal display device of portable type is required to be as minimal as possible in power consumption and hence as low as possible in driving voltage. If the driving voltage is to be lowered, it is important for the display device to have an electrooptical property exhibiting a low threshold voltage. This problem of lowering the threshold voltage of the display device is serious in particular for a liquid crystal microcapsule of guest-host mode where a dichroic dye is dissolved into a liquid crystal and the liquid crystal is microcapsulated. Namely, it is considered that in order to realize a clear color display, a voltage of as high as 20V to 50V is required.
As explained above, in order to realize a bright and clear color display in a display of guest-host mode, the solubility and dissolution stability of the dye to the liquid crystal are required to be sufficiently improved to such an extent that make it possible to realize such a high absorbancy of liquid crystal that is required to represent a clear color. If the concentration of dye is low, it would be impossible to obtain a sufficient color display. If the thickness of cell is increased to compensate this insufficient color display, it will not only invite an increase in power consumption, but also retard the response speed of color liquid crystal to an electric field.
With a view to improve the solubility of dye to a liquid crystal, there has been tried to employ a dye having a flexible substituent group such as a long chain alkyl group. The employment of such a dye is effective in some degree in improving the solubility of dye, but invites a problem that the solubility of the dye is highly dependent on temperature. For example, if such a dye exhibits a very small solubility at 5.degree. C. as compared with the solubility thereof at 20.degree. C., the dye would be precipitated at 5.degree. C. even if the solubility of the dye at 20.degree. C. is sufficiently high. Therefore, such a dye can hardly be employed as a display device.
A large number of dichroic dyes have been synthesized and studied for using them in a liquid crystal display device of guest-host mode, but none of them have been found satisfactory as yet in solubility and in dissolution stability under changes in temperature.
Meanwhile, in a liquid crystal display device of guest-host mode as mentioned above, if the concentration of dichroic dye in a liquid crystal is increased for obtaining a sufficient absorbency, it will invite a lowering in retention and an increase in viscosity of a color liquid crystal, thereby deteriorating the electrooptical property of the liquid crystal, thus resulting in a lowering of contrast and an increase in threshold voltage.
Furthermore, if the concentration of dichroic dye in a liquid crystal is increased, the dichroic ratio and the degree of orientational order will be also lowered, thus deteriorating the contrast of the liquid crystal.